I downloaded a new TI-84 app for iPhone and they are using an exact copy of all error messages for all commands from ti-basic.

The copied all error conditions into their app without naming the source.
Check this as an example:

dropbox link: /s/zxtny1o6khopywg/IMG_5931.PNG

I normally wouldn't sneak on something like that but this developer already violated my intellectual property rights.
I just don't like that they make lots of money by just copying the content they need into an app and sell it.

Fortunately, they removed my content after I informed Apple about it.
Google for "App Store Content Dispute" to do that.

If you gave us the specific app, we could probably look at it better, but as far as I see, there is not harm done. An emulator literally emulates a real calculator, which means the same functions, same errors, etc.

Did the app ask for a ROM image? If the app asked for a ROM image, then that means that they are not distributing a ROM with the emulator, therefore it is okay.

As for the error codes, Texas Instruments would be who you want to talk to (not us)…if they disagreed with people using their content. However, they are okay with people using their content for educational and business use (as you mentioned the app being sold). TI is in no way responsible for anything bad happening. That's why we're able to have that information on this site, otherwise, this site would not be legal/okay.

In short, if the app asked for a ROM image and did not distribute one to you, it's okay. While it would be great to reference TI, it's not required.

1. I'm sorry, I had to remove the link to the app because I wasn't allowed to post links and then forgot to mention the name.
The app is named "Graphing Calculator +" by "incpt.mobis" (Apple ID: 1279565050".

2. The app isn't an emulator, it is a "new" calculator, which just looks exactly like a TI-84 with the same functions and commands. It doesn't ask for a ROM image. You pay $3.99 and get the full calculator, which is basically a TI-84.

3. I didn't mean the error message itself, moreover the description of it.
For example this error message of the "identity( command":ERR:INVALID DIM occurs if the size is not an integer 1-99. In practice, however, even identity(37 requires more free RAM than the calculator ever has.
It would be ok to use ERR:INVALID DIM only, but they copied the explanation of the error message 1:1 into their app. I would say that's a copyright infringement of the owner of this website?

Here are some more screenshot and also a screenshot of the TI basic website.
You have to search for a specific command on TI basic and then there are different error descriptions for each command.
Compare the error description of the app and the TI basic website.
dropbox.com /sh/qk7vn3ayw1wtnw7/AACm3Cuq9OaCSOQZmsqSF3nDa?dl=0
-> delete the space between dropbox.com and the rest of the link

In addition, the app is probably using a ROM to function. Either way, the app is not following legal terms. Your best bet is to report the app. I'll notify the rest of the staff and make sure this is the correct approach.